Air separation plants separate atmospheric air into its primary constituents: nitrogen and oxygen, and occasionally argon, xenon and krypton. These gases are sometimes referred to as air gases.
A typical cryogenic air separation process can include the following steps: (1) filtering the air in order to remove large particulates that might damage the main air compressor; (2) compressing the pre-filtered air in the main air compressor and using interstage cooling to condense some of the water out of the compressed air; (3) passing the compressed air stream through a front-end-purification unit to remove residual water and carbon dioxide; (4) cooling the purified air in a heat exchanger by indirect heat exchange against process streams from the cryogenic distillation column; (5) expanding at least a portion of the cold air to provide refrigeration for the system; (6) introducing the cold air into the distillation column for rectification therein; (7) collecting nitrogen from the top of the column (typically as a gas) and collecting oxygen from the bottom of the column as a liquid.
In certain cases, the air separation unit (“ASU”) can be used to supply one of its air gases to a nearby pipeline (e.g., an oxygen or nitrogen pipeline) in order to supply one or more customers that are not located immediately near the ASU. In a typical ASU supplying a local pipeline, it is common to use a process configuration utilizing an internal compression (pumping) cycle, which in the case of an oxygen pipeline, means that the liquid oxygen produced from the lower pressure column is pumped from low pressure to a higher pressure than that of the pipeline and vaporized within the heat exchanger, most commonly against a high pressure air stream coming from a booster air compressor (“BAC”) or from the main air compressor (“MAC”). As used herein, a booster air compressor is a secondary air compressor that is located downstream of the purification unit that is used to boost a portion of the main air feed for purposes of efficiently vaporizing the product liquid oxygen stream.
Under normal conditions, the ASU feeding oxygen to the oxygen pipeline is designed to produce oxygen at a constant pressure. This is because ASUs operate most efficiently at steady state conditions. However, pipelines do not operate at constant pressures. For example, it is not uncommon for an oxygen pipeline to operate between 400 and 600 psig (i.e., about a 200 psig pressure variance) during a single day. This can occur due to variable customer demand, variable supply to the pipeline, and/or variable pipeline hydraulics.
In the prior art known heretofore, it is customary to design the ASU to provide the oxygen gas at a constant pressure that is above the highest pressures expected for the pipeline. In order to address the problem associated with pipeline pressure variance, it is customary to let down the pressure of the gaseous oxygen across a control valve to approximately match the pressure of the pipeline just prior to introducing the oxygen gas to the pipeline. However, this method suffers from inefficiencies anytime the pipeline pressure is below that of the design pressure of the ASU. Therefore, it would be advantageous to provide a method and apparatus that operated in a more efficient manner.